Typical military aircraft often have an upstream air inlet that directs air back to the engine. As the aircraft moves through the air the boundary layer of air closest to the surface of the aircraft experiences drag. This drag reduces the speed of the air in the boundary layer with respect to the free stream of air farther away from the surface of the aircraft. The boundary layer may also have turbulence or other distortions due to interactions with the aircraft surface.
A boundary layer diverter is a passageway that directs the boundary layer of air away from the inlet, and thus, limiting low energy air from entering the engine. With a boundary layer diverter in place, the free stream air entering the inlet and into the engine will have a more uniform profile of pressure and speed across the inlet than would be the case without the diverter. Generally this will result in more efficient engine performance and decreased life cycle cost.
An aircraft with a boundary layer diverter may be less survivable and more observable than an equivalent aircraft without a diverter. This may be particularly true if the entrance to the passageway is visible from below or in front of the aircraft. The potential increase in observability may correspond to a decrease in survivability.
Thus, aircraft designers may be faced with the following choices: To design an aircraft whose engines perform as efficiently as possible all the time or to design an aircraft to be more survivable and less observable all the time.